Inductive or capacitive detector

ABSTRACT

A proximity detector comprising a reference oscillator and a measurement oscillator the frequency of which depends on the proximity of an object, particularly a metal object.  
     A detection counter value A, which is controlled by the frequency F of the measurement oscillator 10, is compared to a set point counter value B to create the output signal. The set point counter value B 0 , which is determined during the calibration operation, is corrected by a quantity C that depends mainly on the difference of the measured temperature θ compared to the calibration temperature θ 0 .  
     Application to the detection of ferrous or nonferrous objects by universal or selective inductive detectors or capacitive detectors.

[0001] The present invention relates to an inductive or capacitive proximity detector that is particularly capable of detecting ferrous or non-ferrous metals, or a capacitive proximity detector comprising an oscillator capable of transmitting a measurement signal the frequency of which is a function of the proximity of an object to be detected, and a digital processing circuit that controls an output of the detector as a function of the frequency of the measurement oscillator.

[0002] This type of proximity detector is described in German patent DE 32 04 405. In this detector two oscillators are provided that oscillate at the same frequency, one of the oscillators being inducted and the other oscillator not being inducted by the object to be detected. The oscillator outputs are applied to respective counters such that the counting difference observed in the presence of a target is used by a logic circuit that commutes an output signal.

[0003] British patent GB 1 510 034 describes an inductive proximity detector comprising a variable frequency oscillator and a reference oscillator that operate with a detection counter and a reference counter, said reference counter defining a counting interval for the detection counter.

[0004] The known proximity detectors do not take into account variations or drifts affecting the components of the processing circuit, particularly depending on the temperature, which therefore results in insufficient measurement accuracy and reliability.

[0005] The aim of the invention is to detect objects, particularly ferrous or non-ferrous metal objects, using frequency processing that ensures excellent measurement accuracy.

[0006] According to the invention the set point counter value is stored in a memory associated with the digital processing circuit and it is correctable particularly according to the temperature, and a calibration counter value is introduced into the memory when the detector is calibrated in the presence of an object located at the nominal range, the set point counter value being obtained by correcting the calibration value according to the temperature.

[0007] Preferably, the set point counter value is corrected cyclically, for example at each measurement or every n measurements, n being a predetermined number, according to the ambient temperature measured at the time.

[0008] The following description is of a non-limitative embodiment of the invention and refers to the attached figures where:

[0009]FIG. 1 is a diagram of the circuit of an inductive proximity detector of the invention.

[0010]FIG. 2 is a schematic diagram of the casing of a detector of the invention.

[0011]FIG. 3 is an synoptic diagram of the operating mode of the detector in FIG. 1.

[0012]FIG. 4 is an synoptic diagram of the calibration of the detector.

[0013] The inductive detector of FIG. 1 comprises a measurement oscillator 10 that transmits an oscillation to a shaping circuit such that it produces a pulsed measurement signal S the frequency F of which depends on the proximity of a ferrous or non-ferrous metal object M. The detector is either universal, i.e. capable of detecting objects irrespective of whether they are ferrous or non-ferrous at the same distance, or selective, i.e. only capable of detecting ferrous or non-ferrous objects.

[0014] A reference oscillator 11 is provided in the detector that, once it has been shaped, is capable of transmitting a pulsed reference signal Sr the frequency F_(r) of which is not dependent on the object. Oscillator 10 oscillates, for example, at a measurement frequency F of approximately 1 MHz that varies when it approaches a metal object and oscillator 11 oscillates, for example, at a fixed frequency F which may be several hundred kHz or several MHz according to the applications and which in the embodiment considered is 8 MHz. Frequencies F and F_(r) can also be equal. The detector comprises a processing circuit 12 equipped with a programmed logic circuit, microcontroller or similar circuit 13 on one clock input 13 a to which measurement signal S is applied and on one counter input 13 b to which reference signal S_(r) is applied. Counter input 13 b is connected to a counter 14 of microcontroller 13. The microcontroller has a calibration input 13 c and a temperature capture input 13 d. A rewriteable memory 15, for example of the EEPROM type, is connected to an input/output 13 e of the microcontroller and an output 13f of the microcontroller carries a control signal S_(out) from the output of the detector.

[0015]FIG. 3 shows the main operating program of the detector. Measurement pulses S that match measurement frequency F are counted to create a counting window of width T defined in an operation 20; width T of this window is equal to a given constant number of clock periods of the microcontroller and is in inverse proportion to the frequency of S. Width T is equal to K/F, K being a constant. Counter 14 receives reference signal S_(r) from input 13 b and counts in stage 21, in window T which is controlled by measurement signal S, the number of pulses, for example several thousands, of reference signal S_(r). This yields a counter value A that is read in a stage 22. In one version the pulses from measurement signal S are also counted in a window determined according to reference frequency F_(r).

[0016] Counter value A is compared in a stage 23 with a correctable set point value B that is created as described below and that is updated cyclically, for example by being modified every measurement or, to save processing time, every n measurements, n possibly being equal to 3. An output control signal is emitted (operation 24) as a result of the comparison.

[0017] The temperature of the detector is captured in a stage 30 in which a numerical value 0 is defined using an analogue temperature detector device; this value is compared in a stage 31 with a value θ₀, that is defined in a calibration phase described below. The result is a deviation calculated deviation E=θ−θ₀. The value B₀ is also established during the calibration phase. In a subsequent stage 32 in which a correction value C is established the microcontroller searches in a correction table 16 for value C that is deviation E that was calculated. In a subsequent stage 33 the microcontroller calculates the sum B=B₀+C and this value B is used in stage 23 as described above.

[0018] The software that enables the two values B₀, θ₀ to be obtained in the calibration phase will now be described with reference to FIG. 4.

[0019] A target is positioned at the required detection distance (range) of the detector, then a calibration triggering order is formulated by the operator, preferably by earthing input 13 c of the microcontroller.

[0020] The calibration triggering starts an operation 40 for defining the measurement window T₀=K/F₀, K being a constant and F₀ being the frequency of the oscillator that measures the range. The counter starts a counter operation 41 of the pulses of reference signal S_(r) from the reference oscillator 11 after it has been shaped. The counter value B₀ obtained at the end of the measurement window T₀ is read in an operation 42 and stored in EEPROM 15.

[0021] The calibration is continued by an operation 43 in which the temperature is measured and the matching analogue value is converted into a digital value θ₀ that is also stored in EEPROM 16. Both B₀ and θ₀ are then taken into consideration in the current measurements, as seen above. The calibration described above overcomes the dispersions and variations that affect the components of the circuit. The switching hysteresis of the detector is included in the measurement software to correct the set point counter value B. Calibration is performed initially and whenever it may be required.

[0022] An example of a compact embodiment of the detector is shown schematically in FIG. 2. Casing 50 of the detector houses two oscillators 10, 11 that have LC oscillating currents. Inductor L₁₁ of oscillator 10 is an air inductor with or without a ferrite core. Inductor L₁₁ of oscillator 11 is located on printed circuit board 51 that includes processing circuit 12 and output circuit 17. Capacitors C₁₀, C₁₁ of both oscillators belong to the same capacitor network located on printed circuit board 51 in order to present drifts in temperature that are as close as possible to one another.

[0023] The detector described above is a universal type detector that detects from ferrous and non-ferrous objects alike at the same distance. it can also be a selective type detector and frequency F_(r) is then much lower, for example approximately several tens of kHz. The invention also applies to capacitive detectors. 

1. An inductive or capacitive proximity detector comprising: a measurement oscillator (10) capable of transmitting a pulsed signal (S) the frequency (F) of which is a function of the proximity of the object to be detected, a reference oscillator (11) capable of transmitting a pulsed signal (S_(r)) the frequency (F_(r)) of which does not depend on the object to be detected, a digital processing circuit (12) that transmits an output signal from the detector that is a function of the signal transmitted by the measurement oscillator and with which a counter (14) and a set point counter value B are associated, the counter (14) transmits a detection counter value (A) that is a function of the number of pulses received from one of the oscillators (11, 10) measured in a counting timing window (T) that is dependent on the frequency (F) of the other oscillator (10, 11), the detection counter value (A)is compared with the set point counter value (B) to create the output signal, characterized in that: the set point counter value (B) is contained in a memory (15) associated with the digital processing circuit (12) and is correctable particularly depending on the temperature (θ), a calibration counter value (B₀) is introduced into the memory when the detector is calibrated in the presence of an object located within the nominal range, the set point counter value (B) being obtained by correcting the calibration value as a function of the temperature (θ).
 2. Proximity detector of claim 1 characterized in that the set point counter value (B) is modified cyclically at the end of a number of predetermined measurements.
 3. Proximity detector of claim 1 characterized in that the counting timing window (T) is a function of the frequency of the measurement oscillator (10) and that the digital processing circuit (12) has a counting input connected to reference oscillator (11).
 4. Proximity detector of claim 1 characterized in that: when the detector is calibrated the calibration counter value (B₀) and a reference temperature value (θ₀) are memorized, a current temperature value (θ) is measured when an object is detected, a correction quantity (C) being determined from a correction table as a function of the deviation (θ″θ₀) calculated by the processing circuit (12) between the standard temperature value and the reference temperature value, the calibration value counter (B₀) is assigned to the correction quantity (C) to create the corrected set point counter value (B).
 5. Proximity detector of claim 1 characterized in that the detector is calibrated by means of a learning operation.
 6. Proximity detector of claim 1 characterized in that at the beginning of the operation in which the detector is calibrated a pulse counting window (T₀) is created that is in inverse proportion to the frequency (F₀) of the measurement oscillator (10).
 7. Proximity detector of claim 1 characterized in that the detector is an inductive type detector and such that the measurement oscillator (10) and reference oscillator (11) are LC oscillating currents the capacitors (C₁₀, C₁₁) of which belong to the same capacitor network.
 8. Proximity detector of claim 1 characterized in that the digital processing circuit (12) and the output circuit are installed on a printed circuit board and that the inductor (L₁₁) of the reference oscillator (11) is provided on the on printed circuit board whereas the inductor (L₁₀) of the measurement oscillator (10) is an air or ferrite inductor. 